1. Field of the Invention
The present invention relates to scanning devices and scanning methods which are used to scan for defects on the surfaces of specimens (e.g., silicon wafers, etc.) and to acquire corresponding specimen pictures using an electron beam.
2. Description of the Related Art
Accompanying the heightened integration of LSI in recent years, there was a demand to further raise the sensitivity of detection of defects on specimens such as on silicon wafers, masks, etc. For example, in a 256 megabit DRAM, a detection sensitivity is required to about a 0.1 μm defect dimension with respect to a 0.25 μm pattern dimension. Consequently, a specimen observation and detection device was proposed which used an electron beam, of higher minimum resolving power than the prior art.
For example, in a scanning device described in Japanese Laid-Open Patent Publication No. JP-A-H4-242060, a reflecting electron microscope is disclosed in which, irradiating an electron beam on a specimen surface, the image of reflected (back scattered) from this irradiation region is projected onto a detection surface, and a picture of the specimen is acquired.
Such a reflecting type electron microscope is described with reference to a drawing figure attached to this patent document and which has been identified as “FIG. 16.” In FIG. 16, the electron beam irradiated from the electron gun 71, passing through the irradiation lenses 72, is incident on the central portion of the Wien filter 73. The electron beam at this time has its locus curved by the Wien filter 73, and is perpendicularly incident on the specimen 75 on the stage 74.
When the specimen 75 is irradiated by the electron beam, secondary electrons consisting of reflected electrons are emitted from this irradiation region. The secondary beam is not affected by the deflecting action of the Wien filter 73, and proceeds straight on, to be imaged on the fluorescent plate 77 by means of the imaging lens system 76. At the fluorescent plate 77, the reflected electron image is converted into an optical image. The image of the specimen surface can be observed by imaging in a CCD camera or the like.
However, in a scanning device which detects defective places of a specimen, the whole surface of the specimen is imaged, and then the defective places are detected from the pictures of the whole surface of the specimen; this is the sequence generally adopted to observe the defective places.
In the above-mentioned reflecting electron microscope, the case is considered in a drawing figure attached to this patent document which has been identified as “FIG. 17,” which shows the observation of defective places in a chip of a semiconductor wafer.
Imaging the whole surface of a chip, by first irradiating the region [1] of the chip with the electron beam, the image of this region is imaged, projected on the fluorescent plate 77 (FIG. 16). Next, the stage 74 is caused to move, and region [2] is irradiated with the electron beam, and similarly this region is imaged. Subsequently, region [3], region [4], . . . are successively irradiated with the electron beam, repeating the imaging, and the whole surface of the chip is imaged.
Nevertheless, at this time, before completing the image of region [1], because the stage 74 moves and the region [2] cannot be imaged, the problem was that each region has to be imaged in one step, and a long amount of time was required for imaging the whole surface of the chip.
Moreover, after imaging the whole surface of the chip, the defective places of the device pattern are detected by carrying out picture processing, and observations are made, imaging these places. At this time, it is the case that the defective places are not enlarged for observation.
For example, taking the local region A in FIG. 17 as a defective place, we consider the case of observing this place, enlarged. The focal length of the imaging lens 76 (FIG. 16) changes, and though the local region A may be projected, enlarged, on the fluorescent plate 77, in the case that the local region A is small the enlargement ratio has to be large, bringing about the disadvantage that the observed image became a dark, low contrast image. Moreover, at this time, due to the effect of the aberration of the imaging lens system 76, the observed image became indistinct and the picture quality of the observed images decreased.
In the scanning device described above, operating to image a wide range of regions so as to image the whole specimen surface, because two imaging operations are necessary to operate the observation of a local region such as a defective place, these two imaging operations had to be made compatible.
In particular, in the case of detecting defective places, because the whole surface of the specimen had to be imaged at high speed, speed was regarded as important in the imaging operation. Moreover, in the case of observing defective places, because it would be preferable to obtain pictures of only these places, picture quality was regarded as more important than speed in the imaging operation.
With other types of scanning devices such as those described in U.S. Pat. No. 5,576,833 and which use a rectangular beam having the beam cross section shaped into a rectangular shape, by scanning on the specimen surface while causing movement of the stage, an increased speed of defect detection was provided.
However, in the scanning device described in U.S. Pat. No. 5,576,833, because a specimen picture projected on a MCP (microchannel plate) is acquired by a line CCD sensor, the rectangular beam projected onto the specimen had to be long and slender, corresponding to the shape of the line CCD sensor. Because of this, the area of the specimen image projected onto the MCP became very small, and the problem arose that the lifetime of the MCP was reduced.